An acoustic wave device using a piezoelectric thin-film resonator is used as a filter provided in wireless communication equipment, for example. The piezoelectric thin-film resonator includes a film bulk acoustic resonator (FBAR) and is configured to have a piezoelectric thin-film having opposite surfaces on which upper and lower electrodes are respectively provided. The resonance frequency of the piezoelectric thin-film resonator depends on the thickness of a portion in which the upper and lower electrodes face each other through the piezoelectric thin-film. Such a portion is referred to as a resonance portion. In a case where the upper and lower electrodes are respectively made of different materials, the resonance frequency of the resonance portion depends on the materials as well as the film thickness.
Japanese Patent Application Publication No. 2002-335141 (Document 1) discloses an art in which an addition film is formed on the upper electrode in the resonance portion in order to change the resonance frequency of the piezoelectric thin-film resonator. U.S. Pat. No. 6,657,363 (Document 2) discloses an art of forming an opening in the addition film on the upper electrode in the resonance portion.
According to the art disclosed in Document 1, resonators having different resonance frequencies are realized by forming addition films having different thicknesses. Such addition films are formed by repeatedly performing the process of forming one addition film a number of times corresponding to the number of different resonance frequencies. This increases the production cost.
The art disclosed in Document 2 needs etching for forming an opening in the addition film on the upper electrode. Therefore, it is required that the addition film has etching selectivity to the upper electrode. This requirement limits the range of selection of materials for the addition film. Further, the use of a single-layer addition film leads to a limited range of the film thickness in which the resonance performance is kept and the resonance frequency is adjustable.